By means of steam, hydrocarbons can catalytically be converted to synthesis gas, i.e. mixtures of hydrogen (H2) and carbon monoxide (CO). As is explained in Ullmann's Encyclopedia of Industrial Chemistry, Sixth Edition, 1998 Electronic Release, keyword “Gas Production”, this so-called steam reforming is the most frequently used method for the production of synthesis gas, which subsequently can be converted to further important basic chemicals such as methanol or ammonia. Although different hydrocarbons, such as naphtha, liquefied gas or refinery gases, can be converted, steam reforming with methane-containing natural gas is dominant.
The steam reforming of natural gas is strongly endothermal. Therefore, it is performed in a reformer furnace in which numerous catalyst-containing reformer tubes are arranged in parallel, in which tube the steam reforming reaction takes place. The outside walls of the reformer furnace as well as its ceiling and its bottom are lined or provided with several layers of refractory material which withstands temperatures up to 1200° C. The reformer tubes mostly are fired by means of burners which are mounted on the upper or lower surface or on the side walls of the reformer furnace and directly fire the space between the reformer tubes. The heat transfer to the reformer tubes is effected by thermal radiation and convective heat transmission from the hot flue gases.
After preheating by heat exchangers or fired heaters to about 500° C., the hydrocarbon-steam mixture enters into the reformer tubes after final heating to about 500 to 800° C. and is converted at the reforming catalyst to obtain carbon monoxide and hydrogen. Nickel-based reforming catalysts are widely used. While higher hydrocarbons are completely converted to carbon monoxide and hydrogen, a partial conversion usually is effected in the case of methane. The composition of the product gas is determined by the reaction equilibrium; beside carbon monoxide and hydrogen, the product gas therefore also contains carbon dioxide, non-converted methane and steam. For energy optimization or in the case of feedstocks with higher hydrocarbons, a so-called pre-reformer can be used after the preheater for pre-splitting the feedstock. In a further heater, the pre-split feedstock then is heated to the desired reformer tube inlet temperature.
After leaving the reformer furnace, the hot synthesis-gas product gas is partly cooled in one or more heat exchangers. The partly cooled synthesis-gas product gas subsequently undergoes further conditioning steps which are dependent on the type of product desired or on the succeeding process.
The steam reforming of natural gas is characterized by its high energy demand. The prior art therefore already includes suggestions in which it should be attempted to minimize the demand of foreign energy by an optimized procedure, for example by energy recovery. A so-called HCT reformer tube with internal heat exchange has been presented by Higman at the EUROGAS 90 Conference, Trondheim, June 1990. Said HCT reformer tube comprises an outer reformer tube filled with catalyst and heated from outside, in which the catalyst bed is traversed by the feed gas from top to bottom. In the interior of the catalyst bed, two coiled heat exchanger tubes arranged as double helix and made of a suitable material are provided, through which the partly reformed gas flows after leaving the catalyst bed and in doing so releases part of its sensible heat to the steam reforming process which takes place at the catalyst. Calculations and operational experiments have shown that at a typical inlet temperature of 450° C. into the catalyst bed and at a typical outlet temperature of 860° C. from the catalyst bed, up to 20% of the energy required for steam cracking can be recirculated to the steam reforming due to the internal heat exchange. Furthermore, up to 15% investment costs are saved, since the convection path in the reformer furnace can be designed smaller and less reformer tubes are required. What is disadvantageous, however, is the higher pressure loss due to the longer conduction path of the gas through the helically designed heat exchanger tubes. Furthermore, a corrosion referred to as “metal dusting” becomes noticeable to a greater extent, which will briefly be explained below, since longer portions of the heat exchanger tubes are exposed to the temperature range relevant for the “metal dusting” corrosion.
In many synthesis gas production plants, at higher gas temperatures, in particular in the range from 820° C. down to 520° C. in the gas production plants themselves and in the heat exchangers downstream of the same, corrosion problems occur at the metallic materials used, when a certain CO2/CO/H2O ratio is reached. This applies both to ferritic and to austenitic steels. This removal of material known under the term “metal dusting” leads to a consumption or destruction of the material, and there are only limited possibilities to withstand this corrosion by an appropriate material composition.